ATBD - GHG-CCI

ESA Climate Change Initiative (CCI)
Algorithm Theoretical Basis Document
Version 2 (ATBDv2) – UoL-FP
for the Essential Climate Variable (ECV)
Greenhouse Gases (GHG)
Page 18
Version 2 – Draft 1
18 March 2013
the site altitude is lower than the lowest level of a grid point the pressure is calculated with respect
to the lowest level, where the temperature and molar mass are extrapolated downwards based on
the lapse rate and gradient of the 5 lowest levels above, respectively. The surface pressure for the
site can then be resolved by interpolating the pressures with latitude, longitude and time.
To validate the surface pressure two sites were chosen; Southern Great Plains (SGP)/USA,
and Black Forest/Germany, which are flat and mountainous in topography respectively. The
Atmospheric Radiation Measurement (ARM) program has a central facility in SGP which
takes in situ ground based measurements of surface pressure using the Temperature,
Humidity, Wind and Pressure Sensors (THWAPS) instrument. Figure 3-3 shows that the
surface pressure determined is within 1 hPa of THWAPS observations. Surface Pressure
observations were also made by the ARM mobile facility that visited Black Forest, Germany,
in 2007 with the Surface Meteorological Instrument (MET). Figure 3-2 illustrates that over a
mountainous topography the surface pressure can still be resolved to within 1 hPa. Hence,
the surface pressure a priori can be constrained to less than 0.1%, although we use a 1-
sigma error of 4 hPa on the a priori surface pressure estimate to allow for more difficult
topographies.
Figure 3-2. Validation of a priori surface pressure with in-situ ground based measurements
from ARM instruments in Southern Great Plains/USA and Black Forest/Germany.
3.3.1.4 Temperature and Water Vapour
The ECMWF specific humidity data can be used to generate water vapour volume mixing
ratio profiles using the equation:
where R d and R w are the gas constants in dry and wet air respectively. ECMWF also
provides temperature profiles which, along with the H2O VMR , are interpolated with latitude,